Human skin may contain a range of abnormalities including vascular and pigmented lesions. Although not always dangerous to the individual, such abnormalities are frequently cosmetically troublesome.
Vascular lesions, in particular, may take several manifestations. Common examples are `port wine` stain birthmarks; telangiectasias (spots or vessel lines formed by dilated capillaries or other small blood vessels); and hemangiomas (benign tumors composed of well-formed blood vessels).
Leg telangiectasia, or `leg veins`, are chronically dilated blood vessels visually apparent as red or blue linear or `spider` structures. They may cover extensive or local areas of the leg and are more common in women. Large diameter vessels may cause discomfort, while smaller diameter vessels are more often considered cosmetically unsightly by patients.
Up to 80 million adults in the United States alone are affected by leg veins. It is estimated that 29-41% of women and 6-15% of men worldwide have `abnormal` (visually apparent) leg veins. Most vessels presenting for treatment are less than 1 mm in diameter.
The vessels consist of dilated blood channels in an otherwise normal dermal stroma. The blood channels have a single endothelial cell lining with thickened walls consisting of collagen and muscle fibers. Clinically, these vessels may be categorized as linear, arborizing, spider or papular.
Such dilated vessels may result from pregnancy or the use of progestational agents. A genetic link is usually also present. Some such veins are associated with a high pressure flow from a feeding reticular or varicose vein.
In order to eradicate a leg vein, it is usual to damage the endothelial vessel lining or surgically ligate the vessels. Such surgery is radical and performed on an in-patient basis. Endothelial damage may be induced by means of Sclerotherapy or by the use of light energy on an outpatient basis.
Sclerotherapy is currently the favored method of non-surgical leg vein eradication.
Sclerosing agents have traditionally been employed to damage endothelial cells. Such agents as sodium tetradecyl sulfate, hypertonic saline and polidocanol are injected into large vessels (&gt;1 mm in diameter) and result in death of the endothelium. Several systemic injections to a `feeder` vessel system may result in widespread death of the ectatic vessels.
The use of sclerosing agents is associated with telangiectatic matting (formation of clusters of small vessels) in 35% of patients treated, and with hyperpigmentation (residual brown pigmentary staining) in up to 30% of vessels treated. Other adverse sequelae are possible, including ulceration, edema (blistering) and systemic anaphylactic shock. Vessel recurrence within 5 years has been observed in up to 40% of patients studied. Further, many patients are fearful and resistant to the use of needles.
Hyperpigmentation pursuant to sclerotherapy is particularly troublesome, as it replaces the blue vessels with a brown discoloration which may persist for up to 5 years. This effect results from the catabolism of extravasated blood to hemosiderin, a form of iron deposition, brown in color, which may reside in the proximal dermis for up to 6 months.
Sclerotherapy injection difficulties render sclerotherapy relatively unsuitable for the routine treatment of vessels with diameters of less than 1.0 mm.
Light energy has been utilized for the treatment of cutaneous vasculature.
When use of light is under consideration, one can choose to vary wavelength, pulsewidth or coherence (uniformity). Wavelength will typically be chosen by consideration of the absorption and scattering characteristics of the target tissue layers. The absorption characteristics are typified by several peaks in the visible region of the spectrum, due to target chromophores, together with a monotonic decrease into the infra-red region. The scattering of tissue decreases monotonically through the visible to the near infra-red region and beyond.
Both coherent laser light and incoherent light from a flashlamp-type source offer the potential for high selectivity of treatment. Short wavelength (&lt;500 nm) light is usually not employed, since it is highly scattered in tissue and therefore unable to penetrate to a sufficient depth. Light of a wavelength greater than 500 nm has been employed for the treatment of vascular lesions. The absorption profile of whole blood is shown in FIG. 1. This profile will vary with anatomical location, since blood constitution varies, but can be taken as generally representative.
Vascular diseases characterised by small vessels such as the Port Wine Stain respond well to visible wavelength pulsed laser light from a pulsed dye laser, typically with a wavelength in the 550-600 nm range, which is tuned to a local absorption peak of the intravascular blood. Such light, which is absorbed in the top 0.05 mm of the vessel, can coagulate and thereby thrombose a significant portion of the entire cross section of small vessels (&lt;0.1 mm). Construction of such a pulsed dye laser for dermatology applications has been described previously.
Visible wavelength laser light is less effective on larger diameter vessels (&gt;0.1 mm). Although vessel rupture is possible, this represents a non-optimal mechanism associated with the involvement of only the superficial portion of the vessel, due to the shallow absorption depth of the light. Regrowth of the insufficiently damaged vessels usually occurs under these circumstances. Also, the rupture of the vessel leads to an unsightly post-treatment purpura (`bruising) which can persist for up to 2 weeks. This is not well tolerated by patients. It should be remembered also that dilated vasculature of the extremities is also associated with a different ratio of oxy/deoxygenated hemoglobin, the main absorbing chromophores within the blood. A typical leg vein is filled predominantly with deoxygenated hemoglobin, responsible for its blue color. Oxygenated hemoglobin, as typically found in port wine stains on the face, is bright red in color. The near infra-red absorption characteristics of the two principal blood types is shown in FIG. 2. Both hemoglobin types have equal absorption around 800 nm, rendering absorption independent of chromophore mix (and hence of anatomical location) at this wavelength. The magnitude of the absorption coefficient around 810 nm is well suited to the dimensions of the target vessels.
A further disadvantage associated with existing short wavelengh coherent laser sources such as the pulsed dye laser is their short pulsewidth. With a maximum around 1.5 milliseconds, no time for concurrent conduction of the heat is permitted. A pulsewidth of several tens of milliseconds would be desirable. Also, the high cost and the significant bulk of the componentry associated with such devices are prohibitive factors.
A broadband-emitting incoherent flashlamp light source may offer an alternative approach for the treatment of leg veins. Such a source may utilize a spread of principally infra-red wavelengths (550-1200 nm) most of which exhibit a smaller degree of absorption better suited to larger vessels. A longer pulsewidth of up to 100 milliseconds is also available, permitting concurrent heat conduction through the vessel. As a consequence, the full volume of the vessels may be affected, as required for vascular necrosis.
Clinical results from the use of this class of source are at the preliminary stage and may include an obviation of the hyperpigmentation associated with the shorter wavelength/pulsewidth dye laser since proximal rupture is no longer the mechanism in effect. Adverse effects include the occurrence of gross heating effects, edema and blistering associated with the incoherent light, since incoherent light has poor penetration characteristics in human tissue.
Further, the system is physically clumsy and difficult to use. Also, this incoherent light cannot be easily focused to a spotsize which efficiently overlaps the vessels and hence unaffected tissue is involved in the pathological effects. These disadvantages, taken together, limit the applicability of this technology.
Another manifestation of the incoherent flashlamp based light source relates to the use of a mercury-xenon vapor lamp, with specific emission peaks in the visible portion of the spectrum. This incoherent source will result in gross heating of proximal tissue, with a resultant need for concurrent cooling of the skin. Further, the visible emission spectrum of the lamp lends its use to small vessels found in Port Wine Stains, since the light will be absorbed in the top 0.05 mm of the vessels. Larger leg veins are not cited in this patent for this reason.
FIG. 3 illustrates graphically the effect of tuned visible (.about.580 nm) and near infra-red (700-900 nm) coherent light on small (&lt;0.1 mm) and moderate (0.1 mm&lt;diameter&lt;1.0 mm) sized vessels. This figure illustrates the inherent suitability of visible band light to small vessels and of infra-red band light to moderately sized vessels, since destruction of a significant proportion of the vessel is required. This suitability is harnessed only if pulsewidths of the order of several tens of milliseconds are available, with their concurrent conduction permitting useful proximal vessel wall damage. In this figure, the shading shows the heat generation during the pulse. The denser shading associated with visible light signifies the attainment of high temperatures with associated explosive effects. This heat can be expected to conduct further to affect a peri-vascular tissue volume.
A near infra-red narrow-band coherent laser light source with variable pulsewidth would offer the potential for more thorough coagulation of larger vessels, without the adverse effects attendant with the flashlamp source. Further, such a source would penetrate to the required depth in tissue. An alternative source of near infra-red light and associated treatment method is described in the following sections.